1. Field of the Invention
The present invention relates to a hybrid construction machine which includes an engine, and a motor-generator that is mechanically coupled to the engine to assist the power of the engine.
2. Description of the Related Art
Construction machines are typically equipped with a turbocharged engine including a turbo supercharger. In the related art construction machine including the turbocharged engine, when abrupt load fluctuation by a hydraulic pump occurs, a phenomenon called lug-down occurs, in which the engine speed is temporarily reduced. When lug-down occurs, a governor increases fuel injection quantity and attempts to return an actual speed to a target speed in order to cope with the reduction in engine speed, and such abrupt fuel injection can lead to black smoke generation or fuel deterioration.
The above phenomenon occurs when the abruptly-increased hydraulic pressure load exceeds engine output as a result of the delay of rising of engine output resulting from that it takes time for the boost pressure to rise in the turbocharged engine. Under such circumstances, operation of the construction machine slows down due to an insufficient supply of power to the hydraulic pump, which can cause operator discomfort.
It should be noted that the turbocharger is commonly known as the occurrence of a time lag until it becomes sufficiently functional after the start of supercharging, and such delay in response is referred to as turbo lag. The occurrence of turbo lag is due to the mechanism itself of the turbocharger, and therefore is difficult to eliminate.
Therefore, a technology using a hybrid system which includes a motor-generator as a power source in addition to the engine for compensating for the lack of engine output is known. For example, Japanese Published Unexamined Patent Application No. 2003-28071 discloses the following technology (see Abstract). An engine speed Nr corresponding to optimum torque of a set speed N0 is calculated as a target speed in a controller 9. When load torque of the engine 1 is large and an engine speed N is lower than the target speed Nr, the motor-generator 6 is operated as a motor in accordance with a deviation ANr so as to perform torque assist. When the load torque of the engine 1 is small and the engine speed N is higher than the target speed Nr, the motor-generator 6 is operated as a generator in accordance with the deviation ANr so as to generate electric power and accumulate the electricity in a battery 7. Thus, the engine 1 is controlled to get closer to the optimum operation state.
In the related art according to the Japanese Published Unexamined Patent Application No. 2003-28071 (hereinafter simply referred to as the “related art”), the engine is controlled by a governor characteristic (hereinafter, as appropriate, referred to as “droop characteristic”) having a predetermined inclination such that the engine speed decreases with increasing load. Furthermore, in the related art, the droop characteristic of the engine is fixed, and when the target torque of the engine changes, the engine speed is changed in accordance with the droop characteristic. That is, in the related art, control is performed so that the droop characteristic of the engine is fixed and the speed command of the motor is variable.
Furthermore, since the maximum engine speed at no load is determined by the droop characteristic, it is difficult to reduce the maximum engine speed in the related art, which results in a problem such as a loss due to drag of the hydraulic pump driven by the engine.
This problem will be described in detail by using FIGS. 18 to 20. FIG. 18 is a speed-torque characteristic diagram showing the relationship among a droop characteristic of the engine, a target engine speed, and a target engine torque according to the related art. As shown in FIG. 18A, in the related art, firstly, a droop characteristic line corresponding to a no-load speed D is drawn on the speed-torque characteristic diagram. Next, when target engine torque A is given, an equal torque line (horizontal line) of the target engine torque A is drawn on the speed-torque characteristic diagram. Then, as shown in FIG. 18B, the intersection of two straight lines is determined by one point of an intersection point AD. Finally, as shown in FIG. 18C, target speed NA can be uniquely determined by drawing a perpendicular line from the intersection point AD to the axis representing speed.
Next, operation of the engine when the target engine torque is changed in the related art will be described by using FIG. 19. FIG. 19 is a speed-torque characteristic diagram showing changes in target speed when the target engine torque is changed. As shown in FIG. 19, when a no-load speed D and target engine torques A, B, and C are determined, target speeds NA, NB, and NC can be uniquely determined by drawing perpendicular lines to the axis representing speed from intersection points AD, BD, and CD of respective equal torque lines corresponding to the target engine torque values and a droop characteristic line D corresponding to the no-load speed D.
FIGS. 20A and 20B are time series graphs showing the relationship between the target engine torques A, B, and C and the no-load speed D of FIG. 19. As shown in FIG. 20B, in the related art, when the droop characteristic corresponding to the no-load speed D is determined, the engine speed is changed in accordance with the droop characteristic, so that the value of the no-load speed D becomes constantly the same. Consequently, the engine speed at no load does not fall below the no-load speed D. In the related art, therefore, there are still some problems that need to be addressed, such as that a loss due to drag of the hydraulic pump, etc. occurs; that engine noise cannot be reduced; and that the flow control of the hydraulic pump is complicated because the engine speed is changed in accordance with the target torque.